1. Field of the Invention
The present invention relates to an LED lamp and more particularly relates to an LED lamp that can be used as a white light source for general illumination.
2. Description of the Related Art
A light emitting diode (LED chip) is a semiconductor device that can radiate an emission in a bright color with high efficiency even though its size is small. The emission of an LED chip has an excellent monochromatic peak. To obtain white light from LED chips, a conventional LED lamp arranges red, green and blue LED chips close to each other and gets the light rays in those three different colors diffused and mixed together. An LED lamp of this type, however, easily produces color unevenness because the LED chip of each color has an excellent monochromatic peak. That is to say, unless the light rays emitted from the respective LED chips are mixed together uniformly, color unevenness will be produced inevitably in the resultant white light. Thus, to overcome such a color unevenness problem, an LED lamp for obtaining white light by combining a blue LED chip and a yellow phosphor was developed (see Japanese Patent Application Laid-Open Publication Nos. 10-242513 and 10-56208, for example).
According to the technique disclosed in Japanese Patent Application Laid-Open Publication No. 10-242513, white light is obtained by combining together the emission of a blue LED chip and the yellow emission of a yellow phosphor, which is produced when excited by the emission of the blue LED chip. That is to say, the white light can be obtained by using just one type of LED chips. Accordingly, the color unevenness problem, which arises when white light is produced by arranging multiple types of LED chips close together, is avoidable.
An LED lamp as disclosed in Japanese Patent Application Laid-Open Publication No. 10-56208 may have a configuration such as that illustrated in FIG. 11, for example. In the LED lamp shown in FIG. 11, the inside of a cup 403 that mounts an LED chip 401 thereon is encapsulated with a first resin 405, in which a phosphor is dispersed and which is further encapsulated with a second resin 404. A drive current is supplied to the LED chip 401 by way of a lead 402 and a wire 406. The emission of the LED chip 401 is converted by the phosphor included in the first resin 405 into light having a longer wavelength than that of the emission. The emission of the LED chip 401 and the light converted by the first resin 405 cause color mixture, thereby producing light in a desired color. If the LED chip 401 emits a blue light ray and the phosphor included in the first resin 405 produces a yellow light ray responsive to the blue ray, then the blue and yellow light rays are mixed together to produce white light.
However, the luminous flux of a single LED chip is too low. Accordingly, to obtain a luminous flux comparable to that of an incandescent lamp, a fluorescent lamp or any other general illumination used extensively today, an LED lamp preferably includes a plurality of LED chips that are arranged as an array on the same substrate. An LED lamp of that type is disclosed in Japanese Patent Application Laid-Open Publication No. 2003-124528, for example.
When a lot of LED chips are mounted highly densely on the same substrate, those LED chips arranged are molded together with a resin on the principal surface of the substrate. This is because the manufacturing process can be carried out more easily than otherwise. However, the present inventors discovered and confirmed via experiments that an LED lamp, subjected to such a resin molding process, is likely to have splits or cracks in the interface between the encapsulating resin layer and the substrate due to a heat shock and the encapsulating resin layer is likely to produce leakage. Once such leakage happens, the LED chip deteriorates by absorbing water in the air. As a result, the reliability of the LED chip decreases and its life shortens, thus causing a serious inconvenience in practice.